This invention relates to a Joule effect electric furnace for goldsmith, dental, and the like melting processes.
The Joule effect electric furnaces currently in use employ, as a heating element, a resistor which may be formed from one of several alloys, such as nichrome or more complex alloys like "Kanthal", or even from wire platinum. The use of such alloys affords operating temperatures in the order of 1200.degree. C., whereas platinum raises these temperature capabilities to 1500.degree. C.
To achieve still higher temperatures, graphite resistors have been sometimes used which afford working temperatures in the 1800.degree. C. to 2000.degree. C. range, provided that they are operated under vacuum conditions, since at temperature levels exceeding 1200.degree. C., when operated in an air environment, graphite would wear out quite rapidly by releasing carbon dioxide.
In the above-described furnaces, the heating of the crucible containing the metal or alloy to be melted is carried out directly, i.e. the heat is transferred from the resistor to the crucible until the desired melting temperature is achieved. This construction makes the use of a heating chamber unavoidable, inside which chamber the resistor and crucible are positioned, the chamber requiring a lining of refractory material capable of withstanding the high operating temperatures involved. However, it is not infrequent for the chamber, when subjected to high temperatures, to become shorted owing to the reduction of the necessary refractories from oxide to metals, even where such refractories contain 80% alumina, which is the maximum amount allowable for preparing the refractory slurry.
A failure of the refractory requires considerably prolonged and expensive adaptational steps, which are of course to be carried out by skilled personnel, while the furnace operation must be discontinued for as long as one day, which is generally the time required for the refractory to dry completely.